The emission of nitrogen oxides (“NOx”) from lean-burn engines must be reduced in order to meet emission regulation standards. Conventional three-way conversion (“TWC”) automotive catalysts are suitable for abating NOx, carbon monoxide (“CO”) and hydrocarbon (“HC”) pollutants in the exhaust of engines operated at or near stoichiometric air/fuel conditions.
Engines, especially gasoline-fueled engines, are being designed to operate under lean conditions as a fuel economy measure. Such engines are referred to as “lean-burn engines”. That is, the ratio of air to fuel in the combustion mixtures supplied to such engines is maintained considerably above the stoichiometric ratio so that the resulting exhaust gases are “lean”, i.e., the exhaust gases are relatively high in oxygen content.
Although lean-burn engines provide enhanced fuel economy, they have the disadvantage that conventional TWC catalysts are not effective for reducing NOx emissions from such engines because of excessive oxygen in the exhaust. Attempts to overcome this problem have included operating lean-burn engines with brief periods of fuel-rich operation (engines which operate in this fashion are sometimes referred to as “partial lean-burn engines”). The exhaust of such engines is treated with a catalyst/NOx sorbent (nitrogen oxide storage catalyst) which stores NOx during periods of lean (oxygen-rich) operation, and releases the stored NOx during the rich (fuel-rich) periods of operation. During periods of rich (or stoichiometric) operation, the catalyst component of the catalyst/NOx sorbent promotes the reduction of NOx to nitrogen by reaction of NOx (including NOx released from the NOx sorbent) with HC, CO and/or hydrogen present in the exhaust gas.
WO 2008/067375, for example, discloses a nitrogen oxide storage catalyst having a washcoat layer provided on a substrate, wherein said washcoat layer contains Rh and further elements which form a composition which are active in the abatement of nitrogen oxide by trapping and conversion thereof.
There, however, remains a need for improved nitrogen oxide storage catalysts, in particular with respect to the efficiency in the abatement of nitrogen oxide at lower temperatures, i.e. during the cold-start periods of automotive exhaust gas treatment. Furthermore, in view of the costly platinum group metals contained in nitrogen oxide storage catalysts, there is a need to reduce the amounts thereof which are necessary for meeting emission regulation standards.